Hello Alfonso,

Yes, bilharziasis (or schistosomiasis as it is usually known in this
country) is the most serious human disease in which mollusks are
involved.  Three different species of schistosomes, or blood flukes,
infect humans, and they are a major public health problem in many parts
of the world.  Each of the three species of human schistosomes utilizes
a different family of fresh water snails as an intermediate host.  The
parasite passes through its larval stages in the body of the snail,
culminating in a final larval stage that leaves the snail, and is
free-swimming in the water.  This near-microscopic larva is capable of
penetrating human skin, and people become infected when they enter the
water.  Once in the body, the larvae migrate to the blood stream, where
they mature into full-grown blood flukes.
Other species of schistosomes infect various mammals and birds, and each
fluke species has one or more specific fresh water snails that serve as
its host.  Sometimes the free-swimming larvae of waterbird schistosomes
will burrow into human skin.  They can't infect humans, but they die in
the skin, causing an irritating dermatitis known as "swimmer's itch".
In addition to blood flukes, a number of other flukes (lung flukes,
liver flukes, intestinal flukes) can infect humans, and they all have
snail hosts as part of their life cycle.  The snail families involved
are Hydrobiidae, Thiaridae, Potamididae, Planorbidae, Lymnaeidae, and
Bulinidae.  In areas where the appropriate species of snails are absent,
these parasitic infestations are rare.

The other major area where mollusks affect human health is, as you said,
accumulation of toxic materials by bivalves.  Snails are seldom involved
in this type of problem, since they are not filter-feeders.  Such toxic
materials may include: chemical wastes in particulate form; bacteria
(typhoid, etc.); viruses (hepatitis, etc.); and toxic protozoans
(paralytic shellfish poisoning or "red tide").  I have a lot of material
on these topics, if you would like more.

Now I think I'll go have a bowl of clam chowder.

Paul M.